Cellular wireless communication systems based on wireless communication protocols, such as wideband code division multiple access (W-CDMA), long term evolution (LTE), and the like, have been widely used. A cellular wireless communication system includes many wireless base stations forming many cells. Therefore, in recent years, a large number of wireless base stations have been disposed all around.
If a sufficient number of wireless base stations are not disposed, cells are not formed in a wide range, and a problem arises in which a cellular service area (a cover area) may be limited. As a matter of course, it imposes large cost to dispose a wireless base station. Therefore, techniques for efficiently forming a cell have been great concerns of service providers (wireless communication carriers) in recent years.
Incidentally, in general, a wireless base station includes a baseband unit (BBU) and a radio frequency unit (an RF unit). The BBU performs baseband processing that is processing performed on a baseband signal. In contrast, the RF unit mutually converts a baseband signal and an RF signal from one to another and performs transmission and reception of an RF signal via an antenna. In this case, a baseband signal is a low frequency digital signal. Since there is a limit in performing complex processing on an RF signal that is a radio frequency analog signal, at reception of an RF signal, a wireless base station demodulates the RF signal received via the antenna to a baseband, and then, performs various types of processing thereon. At transmission of an RF signal, the wireless base station performs various types of processing on a baseband signal related to information that is desired to be transmitted, modulates the baseband signal to an RF signal, and then, transmits the RF signal via the antenna.
As described above, in general, a cellular wireless base station forms a cell. In a known cellular wireless communication system, a single wireless base station forms a single cell. Specifically, a known wireless base station has a configuration in which a single BBU and a single RF unit are integrated as one unit and thus is enabled to form a single cell.
In contrast, a configuration in which a single wireless base station forms a plurality of cells was recently introduced. Such a wireless base station includes a single BBU and a plurality of RF units. The BBU and the RF units are physically separated from one another, and are coupled to one another via an optical fiber or the like. Thus, a single wireless base station is allowed to form a plurality of cells (cells of a number corresponding to the number of RF units).
Thus, a configuration in which a BBU is not provided for each cell is allowed, and therefore, a computer resource per cell that is to be prepared in a BBU may be reduced (due to a statistical multiplexing effect). Furthermore, an increase in efficiency of a computer resource in an entire system is allowed.
Note that there are cases where such a configuration as one described above, in which a plurality of RF units is controlled by a single BBU, is called centralized radio access network (C-RAN). Also, there are cases where a BBU in a wireless base station having a C-RAN configuration is specifically called centralized baseband unit (CBBU). Furthermore, there are cases where a device in a wireless base station having a C-RAN configuration, which corresponds to an RF unit, is called a remote radio unit (RRU) or a remote radio head (RRH).
Japanese Laid-open Patent Publication No. 2010-213007, Japanese Laid-open Patent Publication No. 2012-182792, Japanese National Publication of International Patent Application No. 2014-514848, and Japanese Laid-open Patent Publication No. 2007-228213 discuss related art.